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post #219 of Old 03-30-2012
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Re: Full or fin keel?

I am not 100% certain that Paulo and I are in agreement on all of this, in fact I am certain we are not, but that is okay. Through discussion I would expect that we will end up perhaps a little closer in our views. This is my understanding of some of the issues being discussed above.

Starting with the issue of relative dampening force of a deep fin vs a shallower greater area keel. (Trying to avoid the semantics issue) The dampening force generated by a keel rolling through the water is proportional to the area of the keel, and the distance (from the roll axis to the center of the dampening) to the third power. So while a deep fin keel will have much less surface area than a traditional, because it has a significantly longer dimension from the roll axis to the center of the dampening than a more traditional keel it may actually develop greater dampening forces.

The potential of a deep fin keel to produce dampening is somewhat mitigated by the fact that high aspect ratio fin keels of stall more easily than lower aspect keels and so potentially produce less side force per unit of area. That said, these types of discussions are hard to broadly generalize and the specific reality will vary with the specific design of the keel, boat and wave shape. In most larger-wave induced rolls, the side speed of the keels through the water is so great that virtually all keels are effectively stalled, and so the side force per unit of area may actually be the same between a deep fin and a shallower full keel.

Further complicating this discussion is the shape of the keel, shape of the hull/keel intersection, and proximately of the keel to the hull. The more vertical geometry of a deep fin keel would result in the face of the keel operating more perpendicular to the flow of the water. In a stalled state that would produce a higher unit force. The area around the hull is highly turbulent during a roll, and might entrain more air, therefore producing less unit side force, and since a full keel has a larger portion of its keel in this region, it might be expected to produce a relatively lower unit side force. But because there is a gentler curve in this area of a traditional full keel, there is also likely to be less turbulence in the area of the keel to hull intersection, therefore somewhat mitigating the impact of the larger percentage of the full keel operating in this zone.

Another factor that complicates this discussion is that traditional full keel boats tend to (but not always)roll at a slower rate than more modern designs (which tend to be lighter for an equal length). That slower roll rate means less turbulence and so a greater dampening unit force per area, but the keel passes through the water at a slower speed suggesting in reduced unit dampening force per area.

(See why I say this is hard to generalize about and why I said 'may result in greater dampening')

As a word about my personal opinion in these discussions, and in an effort to provide a filter which would help provide a fair minded sense of where my personal prejudices tend, when it comes to offshore cruising boats, I am not a fan of the Open Class style boats that Paulo likes. They are clearly faster than more moderate designs, but that speed comes at a price in terms of motion comfort and self-righting.

While these Open Class based designs have demonstrated an enviable record as race boats, and may even be a reasonable basis for coastal cruisers, I personally prefer more moderately narrow beam boats for distance cruising containing significant amounts of offshore use.

Which brings me to the issue of boats in waves and GBurton's question about what happens when that heel angle is reached? Does the fin keel boat just sit there, frozen in time?

I think that these questions have little to do with the keel type, and everything to do with form stability. Fin keel or not, a high form stability boat will want develop a greater force trying to make it heel parallel to the plane of the surface of wave. All other things being equal, a higher form stability boat will tend to move more in sync with the wave face. (Not a good thing in big waves)

To one degree or another, mitigating against that is the tendency of modern deep fin keels with bulb designs to have a proportionately high righting moment due to very low centers of gravity. And also mitigating against the impact of form stability is that deep fin keel boats tend to develop very high roll moments of inertia relative to their displacements often having roll moments of inertia similar to much heavier displacement boats. That would tend to slow the roll rate some, but of course not as much as would be the case of a more moderate beam design, in other words a boat with less form stability.

As I said earlier, in big waves, a large roll moment of inertia does two things, at the top of the wave, it delays the rotation of the boat relative to the rotational force. A good thing, but at the bottom of the wave, its greater stored kinetic energy, tends to cause it to get out of phase with angle of the wave face and continue to roll as the bottom of the wave flattens out so that there is a greater danger of dipping a spar in the water (never a good thing). At the bottom of the wave, the boat with greater form stability would generate more righting force, remaining in sync with the wave surface and so would be less likely to dip a deck or spar or keep rolling as far as a boat with a high roll moment of inertia and/or less stability.

I am running out of time but I also want to touch on the issue of inverted stability. Pretty much any reasonably normal sailboat of most eras of history, placed in perfectly calm water exactly upside down, will remain so. When you talk about a boat with a large angle of positive stability, it simply means there is a smaller angle at which it is stable inverted. But since it requires wave action to invert almost any reasonably safe design, there is usually some wave action trying to rotate the boat past the point that it again starts to develop positive stability. When you talk about traditional designs, or more moderate modern designs (either with an LPS approaching 150 degrees), this can happen at inverted heel angles as small as 25 to 30 degrees. But when you consider a design like the open class boats, with the keels centered, the limits of positive stability can be down in the 125 degree range.

What is impressive to me about the video is that the inverted Open Class boat with its keel canted, begins to develop positive stability at an angle which appears to be somewhere around 15-20 degrees. That is amazing! But to me, in my personal opinion, having to cant a keel of an inverted boat to right the boat is no way to go cruising, especially if you visualize the potential weight shift in an inverted distance cruiser.

Lastly there is the question of the impact of the mast on righting. Like most of this, this discussion point is complex and situational. As someone said above, the reason that a mast may be seen as being helpful to righting a boat is that in a large enough wave situation that a boat can be knocked into an inverted position, the mast being deeper into still water, acts a very deel keel so that the force of the wave is amplified and there is more force exerted to rotate the boat toward upright. But that is in part offset by the ballast affect great weight of the sails and rig below the surface, and damping resistance of trying to move the sail sidewards through the water once positive stability is achieved.

There is also a thought that few boats will survive a roll to inverted with their rig intact. The flat water test, therefore is supposed to demonstrate what happens in most likely condition, (inverted without rig) and the hardest to re-right (inverted without wave action to re-right). I am not sure that I buy that, but that is my understanding of the rationale.


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Last edited by Jeff_H; 03-30-2012 at 06:44 PM.
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